Turf grass (sod) is a living organism that must be handled properly to ensure its survival when it is removed from one location and transplanted to another. It is cut into slabs that are removed from the field in a serial manner by a harvesting machine which operates continuously. The slabs are cut to a specific transverse width and longitudinal length by a cutting head during the harvesting operation
A conveyor, which may be a chain or belt, carries slabs from the cutting head to a stacking head which picks up and stacks the slabs (e.g. on a pallet). Slabs may be passed to the stacking head flat or in rolls.
When slabs are stacked as rolls, several rolls are accumulated on the conveyor before they are picked up by the stacking head. Similarly, when slabs are stacked flat, one or more slabs are accumulated on the conveyor before they are picked up by the stacking head. If the flat slabs are picked up in pairs, their length is equal to one half of the length of the pallet; otherwise the slabs are the same length as the pallet.
Slabs are cut in widths that permit either two or three slabs to rest on a pallet side by side. For example, a 48 inch wide pallet would accommodate three rows of flat or rolled slabs that are sixteen inches wide, or two rows of flat or rolled slabs that are 24 inches wide. Once the pallet contains the desired quantity of turf, the pallet is dropped in the field to be retrieved by other means.
Current stacking heads move using linear and/or rotary actuators that are hydraulically powered. The hydraulic valves that power these stacking heads respond relatively slowly to control signals. This slow response time limits the speed at which hydraulically powered stacking heads can be used while still providing accurate stacking of the slabs.
Servo-hydraulic control valves may be used to improve the performance of high speed hydraulic systems. However, these servo-hydraulic control valves are significantly more expensive to install because they are manufactured with very small clearances and tight tolerances. The small clearances are sensitive to contaminant particles that require more aggressive filtration to remove than is common on agricultural equipment. As a result, servo-hydraulic valves are generally considered to be too expensive for sod harvesting equipment.
When using hydraulic valves, it is also difficult to suppress the vibrations caused by the movements of the stacking head. For example, to allow the use of some hydraulic valves, such as proportional valves, some harvesters use an additional ground roller made of steel. This roller is pressed down on the ground directly beneath the point where the stacking head picks up the slab from the conveyor. Because of the high stiffness of the roller it transmits some of the stacking head reaction forces directly to the ground. Using this additional roller is undesirable because of its added weight and cost.
Additionally, proportional valves typically do not have built in feedback mechanisms that give the controller information about the valve spool position. As such, it is difficult for the controller to determine when the valve is not functioning properly without adding additional sensors to the system.
When rotary hydraulic motors are used, the angular position of the stacking head cannot be adequately controlled to hold the stacking heads in place when operating on fields that are sloped. For example, when operating on sloped fields, the stacking head can drift out of position leading to stacking errors. If the stacking head drifts out of position when dwelling over the conveyor waiting for a slab to come into position, it can damage the head and the conveyor when it tries to pick up the slab.
An additional difficulty with hydraulic systems is that hydraulic flow losses in plumbing and valves are strongly dependent on temperature. Because of this, hydraulic systems require a significant warm up period to allow the hydraulic fluid to reach a specified temperature. This warm up period wastes fuel.
Linear type hydraulic actuators (cylinders) have internal and external seals that are adversely affected by high velocity moves. The combination of high cycle rates on the order of 2400 per hour and high velocities lead to fairly short lives and the onset of leaking after a few months operation. This typically necessitates the replacement of the actuator on a yearly basis.
In addition to the difficulties of designing a hydraulic based stacking head that can stack slabs quickly and accurately, such systems are also difficult and expensive to maintain. For example, to achieve desired speeds and accuracy, the hydraulic system requires more complex and expensive components. Also, operating the stacking head at such speeds leads to more frequent service and replacement of components.
Another problem that exists with current stacking mechanisms is that, due to the way that the slabs are stacked, the stack tends to become rounded on top. This rounding increases the risk that the stack will tip over or split apart. To address this rounding, current stacking mechanisms, once the pallet is full, lift the entire pallet up against a barrier which compacts the slabs on the pallet. This step of lifting the pallet is slow and requires the harvesting machine to temporarily stop until the full pallet has been compacted, dropped, and replaced.